Department of Chemical Engineering and Materials Science
University of Minnesota
Stoichiometric Control, Transport and Mobility-Limiting Scattering Mechanisms in High-Mobility Epitaxial La-doped BaSnO3 Films
Location: EB1 Room 1011
Friday, September 23rd 2016 - 11:00 am
Metals possessing high oxidation potential (e.g. Sr, Ba) are readily oxidized, whereas those with lower potential (e.g. Sn, Ir, W) require stronger reaction conditions. For ternary oxides such as perovskite oxides (ABO3), a difference in oxidation potentials of metal A and B can make synthesis more demanding as compared to their binary oxide counterparts. For instance, if metal B has a lower oxidation potential than that of metal A, a more severe oxidation condition may be required to achieve full oxidation of B in the presence of A. Modern molecular beam epitaxy (MBE) approaches overcome these challenges to a certain extent by using high oxygen pressure, reactive gases such as ozone or by employing oxygen containing metal-organic precursors. The first two approaches often accompany undesirable consequences in MBE such as metal flux instability due to the surface oxidation, or filament oxidation, or even damage to the vacuum pumps due to high oxygen pressure. The latter approach although works for titanates and vanadates, it is not always possible to find an oxygen-containing metal precursor that is compatible with the MBE system, i.e. those with the high vapor pressure, and thermal stability .
In this talk, I will review the grand challenges of the synthesis of complex oxides and will present our group's effort to address these challenges using a new radical-based MBE approach. Using Stannate (BaSnO3) as a model material system, I will present a detailed growth study of epitaxial, phase-pure, stoichiometric BaSnO3 films using hexamethylditin, (CH3)6Sn2 (HMDT) as a tin precursor, elemental solid source for Sr and Ba, and a rf plasma source for oxygen. We will demonstrate that the reactivity of tin radicals is so strong that it produces phase-pure BaSnO3 films not only with oxygen plasma but also with molecular oxygen suggesting a rather unique MBE approach to grow metal oxides of low-oxidation potential element. Combined with a battery of structural characterization techniques, we will present a comprehensive electrical characterization of La-doped BaSnO3 and will discuss how transport can be influenced by the presence of structural defects such as dislocations, and non-stoichiometry, and dopant concentration. We will also discuss different scattering mechanisms in La-doped BaSnO3, which limits the room temperature electron mobility. Finally, we will present pathways to enhance electron mobilities towards high room temperature mobility oxide heterostructures using defect-managed thin films and interfaces.
Work supported by the NSF, and partially through AFOSR YIP Program.